The NS has 1 GB of "internal storage", but this actually part of its non-removable 16 GB of storage. The N1 has 512 MB of internal storage. As many apps only allow you to install them on internal storage (especially any app that has a homescreen widget because of this bug), you have twice as much space for such apps on the NS. However, you cannot upgrade the 16 GB. The N1 has only 4 GB included, but you can upgrade it. In fact, I had already upgraded it to 16 GB.
The biggest thing that I forgot to mention is the reception (both voice and data) on the NS. To me, the N1 and the iPhone 4 both got similar reception -- and that is horrible. The iPhone 4's poor reception is well documented, but I think the N1 was just as bad. If you consider that T-Mobile's network is a step below AT&T's, you get a nasty combination. The NS has much better reception than either of these phones. For that matter, it is much better than any other iPhone I have used (3G/3GS). I'm talking about consistently getting two more bars than the N1, and much clearer and louder reception on the phone. It seems obvious that the use of metal in the outer casing is the key here. Both the iPhone 4 and N1 use metal to destroy your reception.
Some folks have also asked me about battery life. I would say that the NS is slightly worse than the N1, and about the same as an iPhone 3GS. That makes it definitely worse than the iPhone 4. The most obvious culprit here is the screen. Bigger screen = more juice. I came across this amusing blog about switching to the Nexus S from an iPhone. This included a complaint about using the power widget to turn off Bluetooth, Wi-Fi, and even GPS on the phone in order to save battery life. This was kind of shocking to me. As an iPhone user, I almost never turned on Wi-Fi and kept Bluetooth off except when in the car. I had to do this on the 3GS, or I would not make it through the day on a single charge. In fact one of the things that I love about Android is how much easier it is to do these two tasks than on iOS. In fact I always thought that this was a big miss by Apple, but I guess that most people actually leave that stuff on all of the time? I don't understand how anyone who uses their phone much, especially if they receive lots of push email, could get away with leaving all of that stuff on all of the time and not run out of juice by 3 PM. I never tried to do this with the iPhone 4 and its awesome battery, just because I was so used to managing the power on my device. Similarly, I can't imagine NOT charging my phone when I go to bed every night. Anyways, if I manage the Wi-Fi and Bluetooth and charge each night as usual, then the Nexus S makes it through the day easily for me. If I play a lot of games on it or use it as a mobile hotpost, that changes things noticeably.
Finally, some folks didn't like that the NS does not have the glowing trackball that the N1 did. I liked the trackball on the N1. The LED lights weren't that useful, since I get email constantly. Still if I got a blue light, I would know that I got something from Facebook. The trackball was especially useful when there were small parts of the screen that needed to be tapped on. However, I have not found myself missing this on the NS, partially because the screen is bigger, but mostly because the touch accuracy on the screen is very good. I always found the N1 to be slightly worse than the iPhone 4 (which I think is slightly worse than the iPhone 3GS) when it comes to touch sensitivity/accuracy. I think the NS is a little better than the iPhone 4, about as good as the iPhone 3GS.